Naked Science Forum

General Science => General Science => Topic started by: Alex Ball on 16/09/2008 21:16:45

Title: How fast can you propel a pole of any length?
Post by: Alex Ball on 16/09/2008 21:16:45

Alex Ball  asked the Naked Scientists:

Dear The Naked Scientists,

I was wondering: If you had a pole, of any length, and you were able to move your end at or near the speed of light, how fast would you be able to propel the end of the pole?

Thanks,

Alex B.

What do you think?

Title: How fast can you propel a pole of any length?
Post by: lyner on 16/09/2008 21:51:37

If you gave it a sharp whack (with a very fast hammer) you would launch a shock wave through the rod which would travel at the speed of sound through the rod.
If it was a continuous force you used, you would need to provide an awful lot of energy to accelerate the steadily increasing mass of the rod which you would be collecting in front of the hammer. There would be so much energy that the rod would melt and there would be all sorts of other practical factors which I am sure other posters will add!

Title: How fast can you propel a pole of any length?
Post by: RD on 17/09/2008 01:13:39

Alex's question may be about length contraction  (http://en.wikipedia.org/wiki/Length_contraction) which occurs when objects approach the speed of light.

Title: How fast can you propel a pole of any length?
Post by: lyner on 22/09/2008 00:15:00

Once it had stopped accelerating, it would be shorter by an amount predicted by SR but, before that, you would have to introduce GR changes, too if you wanted to describe the situation in detail . Once it had settled down, the pole would be doing the same speed as you (natch) but there would be an issue as to how much energy you needed to put into the thing in order to get it (and you) up to that speed.